1. Field of the Invention
The present invention relates to a display device using a thin film transistor.
2. Description of the Related Art
Display devices formed using inexpensive glass substrates tend to be prevented from being downsized due to increase in the ratio of a region (frame region) at the periphery of a pixel portion used for mounting a driver circuit to a substrate, as the resolution increases. Accordingly, it is thought that there is a limitation on a method in which a driver circuit formed using a single crystal semiconductor substrate is mounted on a glass substrate, and a technique by which a driver circuit is formed over the same glass substrate as a pixel portion, a so-called system-on-panel is regarded as important. Realization of system-on-panel reduces the number of pins which is formed to connect a driver circuit and a pixel portion, and enables to avoid problems such as decrease in yield due to poor connection between the driver circuit and the pixel portion and low mechanical strength at a connection point using a pin when the driver circuit of a semiconductor substrate is mounted on a glass substrate. Furthermore, realization of system-on-panel enables not only downsizing of a display device but also reduction in cost due to decrease in the number of assembly steps and inspection steps.
There are a scan line driver circuit and a signal line driver circuit as typical examples of the driver circuit included in the display device. A plurality of pixels in one line or in a plurality of lines in some cases is selected at one time by the scan line driver circuit. In addition, the input of video signals to the pixels included in the selected line is controlled by the signal line driver circuit.
It is said that, of the signal line driver circuit and the scan line driver circuit, the scan line driver circuit is relatively easily formed over a glass substrate because the scan line driver circuit can suppress a driving frequency to a low level, compared with the signal line driver circuit. In Reference 1 (Yong Soon Lee, et al., “Advanced TFT-LCD Data Line Reduction Method”, SOCIETY FOR INFORMATION DISPLAY 2006 INTERNATIONAL SYMPOSIUM DIGEST OF TECHNICAL PAPERS, Volume XXXVII, pp. 1083-1086, 2006), a technique is described in which a scan line driver circuit and a pixel portion are formed over a glass substrate by using a transistor by use of an amorphous semiconductor.